1. Field of the Invention
The invention relates to a circuit reducing noise, and in particular to a circuit reducing phase noise of an oscillator.
2. Description of the Related Art
Voltage controlled oscillator is an oscillating circuit having oscillating frequency controlled by input voltage. The oscillating frequency is varied by direct current. The voltage controlled oscillator is interfered to generate phase noise. The main interference sources are a) inductance-capacitor circuit has too small quality factor in inductance; b) the active element MOS (bipolar) and current source generate flicker noise or thermal noise; c) in bias voltage VDD/VSS, noise is generated by other circuit.
Referring to FIG. 1, a conventional inductance-capacitor circuit including a first field effect transistor 70 and a second field effect transistor 71. The gate and drain of the first field effect transistor 70 are cross coupled to the drain and gate of the second field effect transistor 71 respectively to generate negative resistance and continuous oscillation. The oscillation frequency depends on the total inductance-capacitor of the spiral inductance 72, the varactor diode 73 and the parasitic elements of transistor. The signals output by the drain 14 of the first field effect transistor 70 and the second field effect transistor 71 has the same wave-form and amplitude but opposite phase.
Since the quality factor for a typical spiral inductance 72 is about 5 to 12 and generally the field effect transistor has much noise, the inductance-capacitor oscillating circuit generates much phase noise which must be reduced.